theorem Th36:
  --(A"") = (--A)""
proof
  let a;
A1: (-a)" = -a" by XCMPLX_1:222;
  hereby
    assume a in --A"";
    then -a in A"" by Th12;
    then (-a)" in A by Th29;
    then a" in --A by A1,Th12;
    hence a in (--A)"" by Th29;
  end;
  assume a in (--A)"";
  then a" in --A by Th29;
  then -a" in A by Th12;
  then -a in A"" by A1,Th29;
  hence thesis by Th12;
end;
